1. Field of the Invention
The present invention relates to an electromagnetic clutch, for selective transmission of driving power, which is suitably used for operating a compressor in a refrigerating system for an air conditioning apparatus for an automobile.
2. Description of Related Art
In an air conditioning apparatus for an automobile having an internal combustion engine, an electromagnetic clutch is provided for kinematic connection of a compressor in a refrigerating system with a crankshaft of the internal combustion engine via a pulley-belt power transmission mechanism, so that the rotating movement of the crankshaft of the internal combustion engine is selectively transmitted to the compressor. A belt in the pulley-belt power transmission mechanism is also used for transmitting the rotating movement of the engine to other various engine supplementing devices, such a water pump for recirculating engine cooling water and a generator for charging batteries.
In the operation of the air conditioning apparatus, an occurrence of a seizing in the compressor causes its rotating shaft to be locked, which can cause an excessive force to be generated in a belt for a kinematic connection of the crankshaft with the compressor, resulting in a damage in the belt. Such a damage of the belt causes the various engine supplementing units, such as an water pump and a generator, to be stopped, thereby causing the engine to be stopped. Thus, a solution is essential in order to prevent such a situation from being occurred.
In a prior art, in an electromagnetic clutch having a rotor for receiving a rotating movement from the internal combustion engine and an armature which is, due to an electromagnetic force of an electromagnetic coil, attracted to the rotor, causing the armature to be rotated by the rotating movement of the rotor, Japanese Unexamined Patent Publication No. 57-51025 proposes a method for de-energizing the electromagnetic clutch in case of an occurrence of locking. Namely, an occurrence of locking causes the armature to be prevented from being rotated and the rotor to continue its rotating movement while sliding with respect to the armature. Such a sliding movement between the rotor and the armature causes the temperature to be abnormally increased at the point of contact. Thus, the use of a temperature fuse is proposed, which is fused by the above mentioned abnormally increased temperature at the location where a sliding contact occurs between the armature and the rotor. The fusing of the temperature fuse causes the electromagnetic coil of the clutch to be de-energized, so that the rotor is disengaged from the armature, thereby releasing the load in the belt. As a result, the belt is prevented from being subjected to an excessive force.
In the above mentioned Japanese Unexamined Patent Publication No. 57-51025 using a temperature fuse, a spool for holding the electromagnetic coil is fixedly connected to a coil housing, and the temperature fuse is arranged in the spool at a location facing an end surface of the rotor.
However, in the structure of the prior art, the temperature fuse is located on a radially extending side surface of the coil spool at a location intermediate between an outer peripheral portion and an inner peripheral portion. As a result, a reduction in the axial length of the coil spool is inevitable for a length which corresponds to the area occupied by the temperature fuse, resulting in a reduction in the number of turns of the electromagnetic coil. Thus, an increase in an electric power is essential in order to keep a desired value of an electromagnetic sucking force, thereby increasing an electric power consumption.
Furthermore, according to the test by the inventors of the present invention, the prior art also encounters a further problem in that heat at a location around the temperature fuse is emitted to the atmosphere less effectively due to the fact that the temperature fuse is, at a location spaced from the coil housing, covered by a resin member for thermal insulation of the electromagnetic coil and the coil spool. Due to the reduced efficiency in the heat emission, a generation of a heat in the electromagnetic coil during a normal operation of the compressor causes the temperature to be highly increased around the temperature fuse, thereby erroneously operating the temperature fuse. Furthermore, during a locked condition of the compressor, transmission of the increased temperature generated at the sliding contacting portion between the armature and the rotor is prevented due to the existence of the resin member. In other words, the transmission of the heat at the sliding portion is less effective, which causes the time to be increased until the fusing of the temperature fuse, resulting in a reduction in a response speed. The reduction in the response speed causes a seizing to occur, due to the temperature increase, at a bearing in the electromagnetic clutch prior to an occurrence of a fusing of the temperature fuse.